1. Field of the Invention
The present invention relates to a semiconductor integrated circuit. More specifically, the present invention relates to a semiconductor integrated circuit including an overcurrent limiting circuit which varies its output voltage according to variation in threshold voltage of an output MOS transistor.
2. Description of the Prior Art
As a prior art semiconductor integrated circuit including an overcurrent limiting circuit, there are known a first overcurrent limiting circuit in which a gate-source voltage of an output MOS transistor is connected to a Zener diode for control by a Zener voltage and a second overcurrent limiting circuit in which a gate-source voltage is connected to several diodes for control by a forward voltage.
Such semiconductor integrated circuit including an overcurrent limiting circuit is disclosed in Japanese Published Unexamined Patent Application No. Hei 06-091262. FIG. 4 is a circuit diagram of a semiconductor integrated circuit including an overcurrent limiting circuit disclosed in Japanese Published Unexamined Patent Application No. Hei 06-091262.
Referring to FIG. 4, a prior art semiconductor integrated circuit 400 including an overcurrent limiting circuit outputs a constant voltage by a Zener voltage or a forward voltage of diodes (D1, D2 and D3) irrespective of the threshold value of an output MOS transistor (M41).
The operation of the circuit will be described. In the prior art semiconductor integrated circuit 400 including an overcurrent limiting circuit, when the output MOS transistor (M41) is brought to an overcurrent state, the drain voltage of the output MOS transistor (M41) rises so that a second MOS transistor (M42) is brought to the on state. The gate-source voltage of the output MOS transistor (M41) is limited by a forward voltage of the three-stage diodes (D1 to D3). The output current of the output MOS transistor (M41) is limited.
In the prior art semiconductor integrated circuit including an overcurrent limiting circuit, the electric current of the output MOS transistor M41 is varied.
The largest variation cause of the current limiting value of the output MOS transistor M41 is variation in threshold voltage of the output MOS transistor M41. When the performance of the output MOS transistor becomes good, the variation is increased.
In the prior art semiconductor integrated circuit including an overcurrent limiting circuit, when the threshold voltage of the output MOS transistor is varied, the output voltage of a constant voltage circuit is not changed. The current limiting value is largely varied.